The maturation of many peptide hormones is attenuated in carboxypeptidase E (CPE)-deficient fat/fat mice, leading to a slowly developing, adult-onset obesity with mild diabetes. To determine the contribution of the hormones generated from the proglucagon precursor to this phenotype, we studied the tissue-specific processing of glucagon and glucagon-like peptide-1 (GLP-1) in these mice. In all tissues examined there was a great reduction in mature amidated GLP-1. Furthermore, a lack of CPE attenuates prohormone convertase processing of proglucagon in both the pancreas and the intestine. These findings suggest that defects in proglucagon processing together with other endocrine malfunctions could contribute to the diabetic and obesity phenotype in fat/fat mice.
L Friis-Hansen, KA Lacourse, LC Samuelson and JJ Holst
EG Siegel, A Seidenstucker, B Gallwitz, F Schmitz, A Reinecke-Luthge, G Kloppel, UR Folsch and WE Schmidt
Liver cirrhosis is often accompanied by a disturbed carbohydrate metabolism similar to type 2 diabetes. To investigate the severity of the defect in insulin secretion in this form of diabetes, we measured insulin release from isolated pancreatic islets of rats with CCl(4)-phenobarbital-induced liver cirrhosis. Cirrhosis was confirmed by clinical signs, elevated liver enzymes and histology. Fasting venous plasma glucose concentrations were equal in rats with liver cirrhosis and in controls. Plasma insulin and glucagon concentrations were significantly greater (P<0.01) in cirrhotic rats than in control animals. Glucose (16.7 mM)-induced stimulation of insulin release from pancreatic islets revealed a twofold increase in control and cirrhotic rats. Basal and stimulated insulin secretion, however, were significantly lower in cirrhotic animals. The incretin hormone, glucagon-like peptide-1 (GLP-1), has therapeutic potential for the treatment of type 2 diabetes. Therefore, islets from control and cirrhotic animals were incubated with GLP-1 in concentrations from 10(-)(11) to 10(-)(6) M. GLP-1 stimulated insulin release in a concentration-dependent manner. In islets from cirrhotic rats, basal and stimulated insulin secretion was blunted compared with controls. These data show that the hyperinsulinemia observed in liver cirrhosis is not due to an increase of insulin secretion from islets, but could be explained by decreased hepatic clearance of insulin. GLP-1 may ameliorate diabetes in patients with liver cirrhosis.
U Ritzel, U Leonhardt, M Ottleben, A Ruhmann, K Eckart, J Spiess and G Ramadori
Glucagon-like peptide-1 (GLP-1) is the most potent endogenous insulin-stimulating hormone. In the present study the plasma stability and biological activity of a GLP-1 analog, [Ser]GLP-1(7-36)amide, in which the second N-terminal amino acid alanine was replaced by serine, was evaluated in vitro and in vivo. Incubation of GLP-1 with human or rat plasma resulted in degradation of native GLP-1(7-36)amide to GLP-1(9-36)amide, while [Ser]GLP-1(7-36)amide was not significantly degraded by plasma enzymes. Using glucose-responsive HIT-T15 cells, [Ser]GLP-1(7-36)amide showed strong insulinotropic activity, which was inhibited by the specific GLP-1 receptor antagonist exendin-4(9-39)amide. Simultaneous i.v. injection of [Ser]GLP-1(7-36)amide and glucose in rats induced a twofold higher increase in plasma insulin levels than unmodified GLP-1(7-36)amide with glucose and a fivefold higher increase than glucose alone. [Ser]GLP-1(7-36)amide induced a 1.5-fold higher increase in plasma insulin than GLP-1(7-36)amide when given 1 h before i.v. application of glucose. The insulinotropic effect of [Ser]GLP-1(7-36)amide was suppressed by i.v. application of exendin-4(9-39)amide. The present data demonstrate that replacement of the second N-terminal amino acid alanine by serine improves the plasma stability of GLP-1(7-36)amide. The insulinotropic action in vitro and in vivo was not impaired significantly by this modification.
MD Robertson, G Livesey, LM Morgan, SM Hampton and JC Mathers
Glucagon-like peptide (7-36) amide (GLP-1) is an incretin hormone of the enteroinsular axis released rapidly after meals despite the fact that GLP-1 secreting cells (L-cells) occur predominantly in the distal gut. The importance of these colonic L-cells for postprandial GLP-1 was determined in healthy control subjects and in ileostomy patients with minimal small bowel resection (<5 cm). Subjects were fed a high complex carbohydrate test meal (15.3 g starch) followed by two carbohydrate-free, high fat test meals (25 g and 48.7 g fat respectively). Circulating levels of glucose, insulin, glucagon, glucose insulinotrophic peptide (GIP) and GLP-1 were measured over a 9-h postprandial period. For both subject groups the complex carbohydrate test meal failed to elicit a rise in either GIP or GLP-1. However, both hormones were elevated after the fat load although the GLP-1 concentration was significantly reduced in the ileostomist group when compared with controls (P=0.02). Associated with this reduction in circulating GLP-1 was an elevation in glucagon concentration (P=0.012) and a secondary rise in the plasma glucose concentration (P=0.006). These results suggest that the loss of colonic endocrine tissue is an important determinant in the postprandial GLP-1 concentration. Ileostomists should not be assumed to have normal enteroinsular function as the colon appears to have an important role in postprandial metabolism.
J Schirra, P Leicht, P Hildebrand, C Beglinger, R Arnold, B Goke and M Katschinski
Twelve patients with non-insulin dependent diabetes mellitus (NIDDM) under secondary failure to sulfonylureas were studied to evaluate the effects of subcutaneous glucagon-like peptide-1(7-36)amide (GLP-1) on (a) the gastric emptying pattern of a solid meal (250 kcal) and (b) the glycemic and endocrine responses to this solid meal and an oral glucose tolerance test (OGTT, 300 kcal). 0.5 nmol/kg of GLP-1 or placebo were subcutaneously injected 20 min after meal ingestion. GLP-1 modified the pattern of gastric emptying by prolonging the time to reach maximal emptying velocity (lag period) which was followed by an acceleration in the post-lag period. The maximal emptying velocity and the emptying half-time remained unaltered. With both meals, GLP-1 diminished the postprandial glucose peak, and reduced the glycemic response during the first two postprandial hours by 54.5% (solid meal) and 32.7% (OGTT) (P < 0.05). GLP-1 markedly stimulated insulin secretion with an effect lasting for 105 min (solid meal) or 150 min (OGTT). The postprandial increase of plasma glucagon was abolished by GLP-1. GLP-1 diminished the postprandial release of pancreatic polypeptide. The initial and transient delay of gastric emptying, the enhancement of postprandial insulin release, and the inhibition of postprandial glucagon release were independent determinants (P < 0.002) of the postprandial glucose response after subcutaneous GLP-1. An inhibition of efferent vagal activity may contribute to the inhibitory effect of GLP-1 on gastric emptying.
A Acitores, N Gonzalez, V Sancho, I Valverde and ML Villanueva-Penacarrillo
Glucagon-like peptide-1 (GLP-1), an incretin with glucose-dependent insulinotropic and insulin-independent antidiabetic properties, has insulin-like effects on glucose metabolism in extrapancreatic tissues participating in overall glucose homeostasis. These effects are exerted through specific receptors not associated with cAMP, an inositol phosphoglycan being a possible second messenger. In rat hepatocytes, activation of phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB), protein kinase C (PKC) and protein phosphatase 1 (PP-1) has been shown to be involved in the GLP-1-induced stimulation of glycogen synthase. We have investigated the role of enzymes known or suggested to mediate the actions of insulin in the GLP-1-induced increase in glycogen synthase a activity in rat skeletal muscle strips. We first explored the effect of GLP-1, compared with that of insulin, on the activation of PI3K, PKB, p70s6 kinase (p70s6k) and p44/42 mitogen-activated protein kinases (MAPKs) and the action of specific inhibitors of these kinases on the insulin- and GLP-1-induced increment in glycogen synthase a activity. The study showed that GLP-1, like insulin, activated PI3K/PKB, p70s6k and p44/42. Wortmannin (a PI3K inhibitor) reduced the stimulatory action of insulin on glycogen synthase a activity and blocked that of GLP-1, rapamycin (a 70s6k inhibitor) did not affect the action of GLP-1 but abolished that of insulin, PD98059 (MAPK inhibitor) was ineffective on insulin but blocked the action of GLP-1, okadaic acid (a PP-2A inhibitor) and tumour necrosis factor-alpha (a PP-1 inhibitor) were both ineffective on GLP-1 but abolished the action of insulin, and Ro 31-8220 (an inhibitor of some PKC isoforms) reduced the effect of GLP-1 while completely preventing that of insulin. It was concluded that activation of PI3K/PKB and MAPKs is required for the GLP-1-induced increment in glycogen synthase a activity, while PKC, although apparently participating, does not seem to play an essential role; unlike in insulin signaling, p70s6k, PP-1 and PP-2A do not seem to be needed in the action of GLP-1 upon glycogen synthase a activity in rat muscle.
MA Luque, N Gonzalez, L Marquez, A Acitores, A Redondo, M Morales, I Valverde and ML Villanueva-Penacarrillo
Glucagon-like peptide-1 (GLP-1) has been shown to have insulin-like effects upon the metabolism of glucose in rat liver, muscle and fat, and on that of lipids in rat and human adipocytes. These actions seem to be exerted through specific receptors which, unlike that of the pancreas, are not - at least in liver and muscle - cAMP-associated. Here we have investigated the effect, its characteristics, and possible second messengers of GLP-1 on the glucose metabolism of human skeletal muscle, in tissue strips and primary cultured myocytes. In muscle strips, GLP-1, like insulin, stimulated glycogen synthesis, glycogen synthase a activity, and glucose oxidation and utilization, and inhibited glycogen phosphorylase a activity, all of this at physiological concentrations of the peptide. In cultured myotubes, GLP-1 exerted, from 10(-13) mol/l, a dose-related increase of the D-[U-(14)C]glucose incorporation into glycogen, with the same potency as insulin, together with an activation of glycogen synthase a; the effect of 10(-11) mol/l GLP-1 on both parameters was additive to that induced by the equimolar amount of insulin. Synthase a was still activated in cells after 2 days of exposure to GLP-1, as compared with myotubes maintained in the absence of peptide. In human muscle cells, exendin-4 and its truncated form 9-39 amide (Ex-9) are both agonists of the GLP-1 effect on glycogen synthesis and synthase a activity; but while neither GLP-1 nor exendin-4 affected the cellular cAMP content after 5-min incubation in the absence of 3-isobutyl-1-methylxantine (IBMX), an increase was detected with Ex-9. GLP-1, exendin-4, Ex-9 and insulin all induced the prompt hydrolysis of glycosylphosphatidylinositols (GPIs). This work shows a potent stimulatory effect of GLP-1 on the glucose metabolism of human skeletal muscle, and supports the long-term therapeutic value of the peptide. Further evidence for a GLP-1 receptor in this tissue, different from that of the pancreas, is also illustrated, suggesting a role for an inositolphosphoglycan (IPG) as at least one of the possible second messengers of the GLP-1 action in human muscle.
J C Parker, K S Lavery, N Irwin, B D Green, B Greer, P Harriott, F P M O’Harte, V A Gault and P R Flatt
Introduction Glucose-dependent insulinotrophic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are important gastrointestinal-releasing hormones involved in the regulation of postprandial nutrient homeostasis ( Meier et al
K Kauter, M Ball, P Kearney, R Tellam and McFarlane JR
Leptin, a recently discovered hormone secreted mainly from adipose tissue, was first described as a regulator of adiposity, food intake and energy metabolism. It is now apparent that leptin physiology is much more complex and is likely to play an important role in many other systems including reproduction, haematopoiesis and immunity. Leptin levels have been shown to be well correlated with body fat in both humans and rodents, with administration of exogenous leptin to rats and mice resulting in loss of body fat. Leptin is, therefore, likely to be an important humoral signal to the central nervous system on body composition and regulation of food consumption. Due to the limited cross-reactivity of leptin from other species in the current assays for leptin, physiological research on leptin has, to a large extent, been restricted to rodents and humans. The aim of this study was to develop a leptin immunoassay suitable for use with sheep, enabling the investigation of the basic physiology of leptin in an animal larger than rats or mice, thus allowing repeated blood sampling. Using this assay we investigated the short-term effects of insulin, adrenaline and glucagon (all modulators of blood glucose) on plasma leptin levels. Antiserum to bovine recombinant leptin (brLeptin) raised in chickens was used to develop a competitive ELISA. Using brLeptin as standard, the assay has a sensitivity of 0. 5 ng/ml with inter- and intra-assay variation of 15% and 7% respectively. The cross-reactivity of human recombinant leptin was 36.5%, while mouse leptin showed no cross-reactivity. Plasma samples from ewes, male castrate animals and rams (n=4-5) diluted in parallel to the standard with mean leptin concentrations of 6.0+/-2. 9, 3.3+/-0.4 and 3.1+/-1.3 ng/ml respectively. Leptin levels in rams were significantly lower than in ewes. The non-significant difference in leptin levels between rams and male castrate animals suggests that testosterone may not be responsible for the lower levels of leptin. Four groups of 3-4 ewes were given intravenous insulin (1 iu/kg), adrenaline (65 microg/kg), glucagon (24 iu/kg) or saline. Blood samples were taken at 1, 3, 5, 10, 20, 30, 60, 90 and 120 min after injection. As expected, glucose levels declined within 10 min of the insulin injection and rose after 3 min following both adrenaline and glucagon injections. Leptin levels, however, remained relatively unchanged for the 2 h following the treatments. Finally, a bolus intravenous dose of glucose (240 mg/kg) was given and sequential blood samples taken. Despite plasma glucose levels rising to over 200 mg/dl, leptin levels did not significantly change over the three hours following treatment. These data indicate that plasma leptin levels in sheep, in contrast to rodents, are not responsive to short-term changes in blood glucose or insulin, as has been shown in humans.
E Bojanowska and B Stempniak
To date, glucagon-like peptide 1(7-36) amide (tGLP-1) has been found to affect the neurohypophysial and cardiovascular functions in normotensive and normovolaemic rats. The aim of the present study was to investigate possible effects of tGLP-1 on the mean arterial blood pressure and the release of vasopressin and oxytocin under conditions of blood volume depletion in the rat. In the first series of experiments, the animals were injected i.p. with either 0.15 M saline or 30% polyethylene glycol (PEG). PEG caused an 18% reduction of blood volume 1 h after injection. No significant changes in the mean arterial blood pressure were found in either normo- or hypovolaemic rats during the experiment. tGLP-1 injected i.c.v. at a dose of 1 microg/5 microl 1 h after the i.p. injection increased similarly the arterial blood pressure in normo- and hypovolaemic rats. The plasma vasopressin/oxytocin concentrations were markedly elevated in hypovolaemic animals and tGLP-1 further augmented the release of both hormones. In the second study, hypovolaemia was induced by double blood withdrawal. The haemorrhage resulted in a marked decrease of the mean arterial blood pressure and in the elevated plasma vasopressin/oxytocin concentrations. tGLP-1 injected immediately after the second blood withdrawal increased the arterial blood pressure. In parallel, tGLP-1 enhanced significantly vasopressin and oxytocin secretion when compared with haemorrhaged, saline-injected rats. The results of this study indicate that tGLP-1 may affect the arterial blood pressure and the secretion of neurohypophysial hormones under pathological conditions brought about by blood volume depletion.